1. Field of the Invention
This invention is related to discrimination membranes, such as thin film composite (TFC) or similar membranes—such as hybrid TFC membranes with nanoparticle additives—including at least one additive which forms halamine compounds when exposed to a halogen such as nitrogen and/or nitrogen releasing containing compounds—and/or other additives including alkaline earth metals, mhTMC and selected other elements, and more particularly to such membranes useful for reverse or forward osmosis, for example to purify water.
2. Background of the Invention
Reverse osmosis membranes, made by interfacial polymerization of a monomer in a nonpolar (e.g. organic) phase together with a monomer in a polar (e.g. aqueous) phase on a porous support membrane are known as TFC membranes and are used where flux and substantial rejection characteristics are required, for example in the purification of water. Various materials have been added to TFC membranes in the hopes of increasing flux without reducing rejection characteristics and have met with limited success. In addition, such membranes are subject to fouling resulting in reduced flux as contaminants, for example from the brackish or seawater to be purified, are believed to build up on the surface of the discrimination layer of the TFC membrane.
TFC membranes became available in about the 1970's and proved commercially successful for some RO tasks. Substantial further development has been done to improve the membranes operational characteristics, including permeability or flux, rejection and fouling resistance.
As shown for example in Chau U.S. Pat. No. 4,950,404, in the late 1980's, a polar aprotic solvent was said to be added to the aqueous solution prior to contacting with an acid halide solution for interfacial polymerization to enhance the operational characteristics of the resultant membrane.
As shown for example in Hirose, U.S. Pat. No. 5,989,426, in the mid 1990's, selected alcohols, ethers, ketones, esters, halogenated hydrocarbons, nitrogen-containing compounds and sulfur-containing compounds were said to be added prior to interfacial polymerization to improve membrane characteristics.
As shown for example in Costa, U.S. Pat. No. 5,693,227, in the mid 1990's, catalysts were said to be added to the aqueous phase to accelerate interfacial polymerization, producing TFC membranes with a smoother surface.
As shown for example in Mickols, U.S. Pat. No. 6,562,266, in about 2000, compounds including phosphorous or other materials were said to be added as a complexing agent to acyl halide before interfacial polymerization. A detectable quantity of the added material was said to be retained in the discrimination membrane as a result of the formation of a complex between the added material and the acyl halide.
The many, and varied, proposed formulations for TFC membranes in some instances have different membrane operational characteristics (such as flux, rejection, and fouling resistance) that make them more suitable for different tasks. These tasks are typically defined by the incoming water quality, plant design, and required product water quality, which together set the operational conditions of the plant (such as required applied pressure). For example, membranes for use in purifying brackish water are conventionally expected to encounter a substantially lower salinity—and be operated at substantially lower pressure—than a membrane useful for seawater desalination.
A typical specification for a TFC membrane used for brackish water RO may require a minimum flux of at least 20 GFD (gallons per square foot per day of filtered liquid) with a minimum of 99.5% salt rejection when operated at a working pressure of about 220 psi on brackish water having an expected salinity of about 2000 ppm (parts per million) or less. On the other hand, a typical specification for TFC membranes to be used for seawater RO may require a minimum flux of 20 GFD with a minimum of 99.5% salt rejection when operated at a working pressure of about 800 psi on seawater having an expected salinity of 32000 ppm or more.
TFC membranes suitable for brackish water use, i.e., at low pressure and salt load, will typically not operate well, when operated at the higher pressure and salinity used for seawater purification. There is no clear cut way to predict their performance under the salinity and operating conditions used for seawater exactly, however it can be said that such membranes typically lose permeability and salt rejection ability as the salinity increases.
As an example, Mickols in example 35 discloses the use of a particular phosphorus compound additive which is said to produce a TFC membrane having membrane operation characteristics, including 19.5 GFD and 99.6% rejection when operated at 150 psi with salinity of 2000 ppm. Although these characteristics may be suitable for reverse osmosis of brackish water, there is no reasonable way to predict how to use such information in the formulation of a TFC membrane suitable for use under the conditions required for in seawater osmosis other than by preparing and testing it.
Based on publicly available information, TFC membranes having membrane properties suitable for reverse osmosis of seawater are conventionally made without the various additives discussed in the prior art, presumably because such additives tend to have deleterious action on TFC membranes rejection.
One common problem with conventional TFC membranes designed for seawater desalination, is quality control and variability in product performance. That is, it is believed that fabricating TFC membranes with predictable flux, rejection and fouling resistance for seawater RO has been so difficult, that fabrication facilities are in some cases unable to prepare a specific product, or need to shift a product's formulation in order to maintain the required membrane properties.
Recently, as disclosed in Hoek et al., WO 2006/098872, WO 2008/057842, and US 2008/0237126, UCLA's Nanomaterials & Membrane Technology Research Laboratory determined that the addition of certain nanoparticles, such as LTA, and other materials could be used to improve TFC membrane operational characteristics at operating pressures, and expected salinities suitable for use in reverse osmosis of seawater.
The use of chlorine as a biostatic agent in water supplies is well known. Conventional RO membranes, however, are typically unstable to chlorine and after exposure to fairly low doses of chlorine in the feed water are often chemically degraded, compromising the membrane's ability to reject impurities. RO membranes are believed to be damaged by chlorine exposure through chemical reaction either at terminal amine groups, or the amide groups making up part of the polymer backbone, of the discrimination layer. As a result, conventional RO membranes typically have chlorine removed from the feed water in an additional step to prevent the membrane damage from occurring. In addition to the cost of this additional step, the removal of chlorine from the feed water leads to membrane areas where bio-growth can occur. As a result, biological fouling of RO membranes is a pervasive problem limiting the performance of RO systems.
What are needed are techniques for fabricating TFC membranes, in particular TFC membranes, suitable for operation at the higher pressures and salinities required for reverse osmosis of seawater—which have higher flux than is achievable from TFC membranes made without such additives. Such desirable TFC membranes must also have a suitably high salt rejection, preferably on the order of about 99.5%.
What are also needed are techniques for preventing reduced flow resulting from the formation of biofilms on the membrane surface, e.g. fouling, and techniques to avoid or minimize degradation of RO type membranes to chlorine by increasing the chlorine stability of the membranes.
It would also be desirable to have increased fouling resistance—as well as higher flux and high rejection rates—when compared to a TFC membrane made the same way with the same chemistry but without the additives.
Still further, it would be desirable to be able to produce such TFC membranes with predictable characteristics and yield on a continuous basis.
Still further, it would be desirable to be able to produce anti-fouling TFC membranes with biostatic surface characteristics that could be recharged with exposure to chlorine or other halogens.
Still further, it would be desirable to produce TFC membranes for use with chlorinated water which were not as subject to degradation by chlorine as conventional TFC membranes.
Still further, it would be desirable to produce anti-fouling TFC membranes for use with chlorinated water which were not as subject to degradation by chlorine as conventional TFC membranes and preferably make such membranes anti-fouling TFC RO membranes which can be recharged to maintain their resistance their anti-fouling effectiveness.
TFC membranes are also needed with such membrane operational characteristics for many other uses beyond seawater purification, including but not limited to brackish water purification, waste water reuse, ultrapure water generation, industrial water treatment, other RO tasks, forward osmosis and pressure retarded osmosis.